That's not a product of having someone of the opposite physical sex "in" you - that's a product of your consciousness not accepting/acknowleding your current physical reality. That can certianly happen in tandem with having headmates of different physical sexes (not genders - last I heard, gender didn't define whether you pissed standing up or sitting down) but they're not the same thing.
While I'm sure it could be confusing, difficult, and even traumatic (which covers the "down" side) I don't really get how it's an "up" as well - unless of course you're in a male body and you keep wandering into female toilets to find flowers, and scented oils, and beautiful ornamental tiling etc . . .